1. Field of the Invention
The present invention relates to shoring devices used mainly in underground mines for supporting the roofs of such mines.
2. State of the Art
A problem in underground mining is stabilizing excavations in highly yielding rock masses. After the excavation is made, the surrounding strata squeezes into the entry, causing rib sloughing, roof sag, and floor heave. The squeeze reduces the area available for ventilation and creates a more formidable escape path. One type of mining susceptible to highly yielding ground conditions is longwall coal mining. The entries created to facilitate longwall mining in many cases are subjected to various load stages. These can include the stage when entry is initially developed, when the longwall face proceeds along the entry during its use as a headgate, and when the adjacent longwall panel is mined and the entry is used as a tailgate.
Traditionally, mine operators have used wood timbers and cribbing to support roofs of the mining excavations. Timbers and cribbing are generally effective in supporting the necessary loading, and cribbing will generally yield when subjected to excessive loading. In this respect, wood timbers and cribbing approach being an ideal roof support. Such ideal supports would be able to handle high loading and yet have the ability to yield uniformly when subjected to excessive loading rather than abruptly fail.
Wood timbers and cribbing do, however, have some serious disadvantages. Although heretofore wood timers and cribbing have been generally inexpensive, wood is becoming more scarce and the cost is ever going higher and higher. In addition, regardless of how wood is used, its strength varies with the individual piece and a structure made from wood, such as a crib, is only as strong as its weakest piece. Further, ventilation in mines is of important consideration, and the bulkiness of wooden cribs and posts reduces the amount of air flow.
It has been recognized that it would be highly desirable to produce a simple substitute post support or elongate shoring device to replace wood timbers. Such an elongate shoring device should desirably have equal or greater load bearing capacity than wood timbers as well as the ability to consistently yield in a uniform manner when subjected to excessive loads. It would be further desirable if such a shoring device would be easily and generally inexpensive to install in a mine.
In the March, 1987 edition of Coal Age, it was reported that the Bureau of Mines had developed a yielding steel post for replacement of wood timbers in underground mines. The post mechanism was a three piece unit consisting of a top and bottom leg made from standard steel pipe and a separate foot bracket. One leg telescopes into the other, and an interference ring is welded to the outside diameter of the smaller pipe. As the two pieces of pipes are forced together longitudinally, the ring engages the outer pipe and causes the larger pipe to flare outwardly and deform radially. The action is similar to the extrusion process for manufacturing seamless pipe.
Unfortunately, however, due to the critical dimensions required by the inside diameter of the larger pipe and the size of the interference ring on the smaller pipe, the costs of the steel pipe shoring units was rather high. Even then, consistency from unit to unit was hard to obtain. The pipes had a tendency to seize and buckle rather than yield under high load. In addition, rather than flare and deform radially outwardly as designed, the outer pipe often failed along its seam line. The flaring, deforming pip system as reported by the Bureau of Mines has not been used commercially to any significant amount.
3. Objectives
A principal objective of the invention is to provide a novel, elongate shoring device constructed from telescoping pipe sections, with a non-compressible solid material contained in a compartment formed between the two telescoping pipe segments that sustains and distributes the load between the two sections of pipe rather than frictional and radial deformation forces acting on the pipe as in the units reported by the Bureau of Mines mentioned previously. The novel shoring devices of the present invention are provided with a relief opening that allows the non-compressible to be ejected from the compartment thereby allowing the pipe sections to yield in a uniform manner longitudinally along the length of the pipe units while continuing to provide maximum load bearing support.
An additional objective is to provide such an elongate shearing device further having additional advantages as follows:
(1) The devices of the present invention are not flammable and are much less bulky than timber crib type shoring. The less bulky devices of the present invention allow more adequate air flow in the mine shaft.
(2) The devices of the present invention are inexpensive, easy to install in the mine, and shipping costs are considerably less than with conventional crib type shoring.
(3) The yield and closure characteristics of the devices of the present invention are uniform and easily controlled.
(4) The overall cost of installing the shoring devices of the present invention is significantly less than the total cost, including materials and extensive labor, realized in installing conventional crib type shoring.
(5) Manufacturing of the shoring devices of the present invention is inexpensive, utilizing commercially available pipe that need not be made to rigid specifications beyond those of ordinary, standard pipe that is used widely and is readily available throughout the world.